Preparing an Insoluble Salt

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 2.41 — Preparing a pure dry insoluble salt from two soluble reactants (bold C statement — Paper 2 only)
• 2.43 — Practical: prepare a pure dry sample of lead(II) sulfate (bold C statement — Paper 2 only) (required practical)

Specification section 2(h) Chemical tests — also taught in Titrations, Preparing a Soluble Salt, Tests for Gases, and for Water, Flame Tests.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

If the salt you want is INSOLUBLE, everything changes, and it changes in your favour. Mix two solutions each containing one of the ions you need, and the insoluble salt forms immediately as a solid suspended in the liquid. That solid is a PRECIPITATE, and the reaction is a PRECIPITATION.

The advantage over the other two methods is that the product separates itself. There is no need to know when the reaction is complete, no titration, and no evaporation — the salt is already a solid and simply has to be collected.

Statement 2.41 — choosing the two solutions

1. Identify the two ions in the salt you want. For lead(II) sulfate, they are Pb²⁺ and SO₄²⁻.
2. Choose a SOLUBLE compound supplying the positive ion. Lead(II) nitrate works, because ALL nitrates are soluble.
3. Choose a SOLUBLE compound supplying the negative ion. Sodium sulfate or magnesium sulfate works, because most sulfates are soluble.
4. Both starting solutions MUST be soluble, or the ions would never be free in solution to meet.
5. Mix them. The insoluble salt precipitates immediately, and the leftover ions — sodium and nitrate here — remain dissolved as spectator ions.
6. The nitrate rule is the useful one in practice: since every nitrate is soluble, a nitrate is almost always the safe choice for supplying the metal ion.

Think of it like two clear liquids that curdle when mixed

Add lemon juice to milk and solid curds appear at once from two liquids that were both perfectly fluid a moment before. Nothing was added that you could see; the two things simply could not coexist in solution. A precipitation is that, arranged deliberately. You choose the two solutions precisely so that the pair of ions that cannot stay dissolved together will meet — and the product collects itself, which is why this is much the easiest of the three preparations.

The reaction is lead(II) nitrate + sodium sulfate → lead(II) sulfate + sodium nitrate, or Pb(NO₃)₂ + Na₂SO₄ → PbSO₄ + 2NaNO₃. The lead(II) sulfate appears as a white solid while the sodium nitrate simply stays dissolved.

The IONIC EQUATION is more revealing and shows how little is actually happening: Pb²⁺ + SO₄²⁻ → PbSO₄. The sodium and nitrate ions take no part at all and are spectator ions, exactly as in a neutralisation.

Statement 2.43 — collecting and purifying the precipitate

1. MIX the two solutions in a beaker and stir. A white precipitate of lead(II) sulfate forms at once.
2. FILTER the mixture. The precipitate is caught on the filter paper as the residue; the solution containing the spectator ions passes through.
3. WASH the precipitate with distilled water while it is still in the filter funnel, and this step is essential.
4. The washing removes the soluble spectator ions — the sodium nitrate solution — which would otherwise dry onto the crystals and contaminate them.
5. DRY the washed precipitate in a warm oven, or between filter papers.

The washing step is the one most often left out and it is where the marks are. Filtering separates the solid from the bulk of the liquid, but the solid comes off the funnel wet with the very solution you were trying to remove. Rinsing with distilled water while it sits in the funnel is what turns a filtered precipitate into a pure one.

Designing a precipitation from scratch

Design a preparation for barium sulfate, an insoluble salt. Name two suitable solutions, explain your choices, and give the ionic equation.

1. The ions needed are Ba²⁺ and SO₄²⁻.
2. For the barium ion, choose BARIUM NITRATE — all nitrates are soluble, so it will certainly dissolve.
3. For the sulfate ion, choose SODIUM SULFATE — all sodium salts are soluble, so it will certainly dissolve too.
4. Mixing them precipitates barium sulfate, which is one of the named insoluble sulfates.
5. The ionic equation is Ba²⁺ + SO₄²⁻ → BaSO₄, with the sodium and nitrate ions as spectators.
6. Notice that the two rules WITHOUT exceptions — all nitrates, and all sodium salts — are what make the design safe.

Answer: Barium nitrate and sodium sulfate, chosen using the two exception-free solubility rules.

That is the general recipe for designing any precipitation: use a NITRATE to supply the metal ion and a SODIUM or POTASSIUM salt to supply the non-metal ion. Both rules have no exceptions, so the starting materials are guaranteed soluble and the only insoluble thing in the beaker is the salt you want.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

The vocabulary of precipitation

Precipitate

An insoluble solid formed when two solutions are mixed.

Precipitation reaction

A reaction in which two soluble compounds react in solution to form an insoluble product.

Residue

The solid caught on the filter paper — here the precipitate itself, which is the product wanted.

Spectator ion

An ion present but unchanged — here the sodium and nitrate ions, which stay dissolved and are washed away.

Statement 2.41 — designing the preparation

1. Identify the two ions in the insoluble salt required.
2. Choose a SOLUBLE compound supplying the positive ion — a NITRATE is always safe, since all nitrates are soluble.
3. Choose a SOLUBLE compound supplying the negative ion — a SODIUM or POTASSIUM salt is always safe.
4. Mix the two solutions; the insoluble salt precipitates immediately.
5. The remaining ions stay dissolved as spectator ions and are removed by washing.

Required practical 2.43 — a pure dry sample of lead(II) sulfate

1. Dissolve lead(II) nitrate in distilled water in one beaker, and sodium sulfate or magnesium sulfate in distilled water in another.
2. Mix the two solutions together in a larger beaker and stir.
3. A white precipitate of lead(II) sulfate forms immediately.
4. Filter the mixture, so that the precipitate is collected as the residue on the filter paper.
5. Wash the precipitate with distilled water while it is still in the filter funnel, to remove the soluble sodium nitrate solution.
6. Repeat the washing so that all the soluble impurity is removed.
7. Dry the precipitate in a warm oven or between two pieces of filter paper.

Variables

Independent (changed) — The two soluble solutions mixed together
Dependent (measured) — The mass of dry lead(II) sulfate obtained

Sources of error

Pb²⁺ + SO₄²⁻ → PbSO₄

Conditions: The ionic equation for the precipitation. The sodium and nitrate ions are spectators and stay dissolved.

The three salt preparations, and when each is used

Model answer [3 marks]

Name two solutions that could be mixed to prepare barium sulfate, and explain why each is suitable. [3]

Barium nitrate and sodium sulfate could be used. Barium nitrate is suitable because all nitrates are soluble, so it dissolves to release barium ions into solution. Sodium sulfate is suitable because all sodium salts are soluble, so it dissolves to release sulfate ions. When the two solutions are mixed, the barium and sulfate ions combine to form insoluble barium sulfate, which precipitates.

Model answer [3 marks]

Explain why the precipitate must be washed with distilled water before it is dried. [3]

After filtering, the precipitate is still wet with the solution that passed around it, and that solution contains the soluble spectator ions such as sodium nitrate. If the precipitate were dried without washing, those soluble compounds would be left behind as solids on the crystals and the sample would not be pure. Washing with distilled water while the precipitate is in the funnel dissolves and carries away those soluble impurities.

Not this: Filtering the precipitate is enough to give a pure sample.

This: The filtered solid is still wet with the solution containing the SPECTATOR IONS, which would dry onto it as a solid impurity. Washing with distilled water in the funnel is what makes the sample pure, and it is a marking point.

Mark-losing trap. BOTH starting solutions must be SOLUBLE, or the ions are never free to meet.

Mark-losing trap. WASH the precipitate with distilled water before drying — this is where the marks are.

Mark-losing trap. Use a NITRATE for the metal ion and a SODIUM salt for the other — both rules have no exceptions.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · Define [1 mark] — What is a precipitate?
2. Grade 7 · Explain [2 marks] — Explain why both starting solutions must be soluble in a precipitation reaction.
3. Grade 8 · Explain [3 marks] — A student filters off a precipitate of lead(II) sulfate and dries it immediately. Explain why their sample is impure.
4. Grade 9 · Describe [6 marks] — Select every statement that belongs in a full-mark description of how to prepare a pure dry sample of lead(II) sulfate.
5. 9+ · Analyse [6 marks] — A student is asked to prepare silver chloride, which is insoluble. They propose mixing silver chloride's two ions by using silver chloride solution and sodium chloride solution. Select every statement that belongs in a full-mark analysis.

The people behind this science

Two ways into the same idea — the one for whom a precipitate was the evidence a new element existed, and the one whose hypothesis let a formula be pinned down at all. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Humphry Davy — the one for whom a precipitate was the evidence a new element existed

A precipitation is chemistry's most direct visual test: something appears, so something was present. Davy relied on exactly that reasoning constantly, because when you have made a substance nobody has seen, precipitation reactions are among the few ways of establishing what it is. He is the right person to ask why a solid appearing from two clear liquids is such good evidence, and what it can and cannot tell you — since a precipitate identifies the ions present and says nothing about how much of them there is.

• “Why does a solid appearing prove something was dissolved?”
• “How did you use precipitation to identify a new substance?”
• “What can a precipitate not tell you?”
• “How do you know the solid is what you think it is?”
• “What did you do when two substances gave the same precipitate?”

Amedeo Avogadro — the one whose hypothesis let a formula be pinned down at all

Writing Pb²⁺ + SO₄²⁻ → PbSO₄ presupposes knowing the formula of lead sulfate, and for the first half of the nineteenth century chemists genuinely could not agree on formulae — water was written HO by some and H₂O by others. Avogadro's hypothesis is what eventually settled them, by making relative molecular masses measurable, though it took Cannizzaro's revival of it in 1860 for chemistry to act on it. He is the right figure for the question of how anyone knows a formula is right.

• “How did chemists decide on a compound's formula?”
• “Why could they not agree for so long?”
• “What did your hypothesis let them measure?”
• “Why did it take fifty years for anyone to use it?”
• “How would you check a formula was correct?”

Carry on through the course

• Previous lesson: Preparing a Soluble Salt
• Next lesson: Tests for Gases, and for Water
• Group 1 — the Alkali Metals — Metals soft enough to cut with a knife, which set fire to water — and get worse as you go down.
• Group 7 — the Halogens — A green gas, a brown liquid and a grey solid — and a trend that runs the opposite way to Group 1.
• The Gases in the Air — Four gases, one of which is doing almost all the chemistry — and how to measure it without ever touching it.
• Combustion and Carbonates — Three things burning in oxygen, one carbonate breaking apart, and the 0.04% gas that changes the climate.
• The Reactivity Series — Putting eleven metals in order by watching what each one will and will not do.
• All of Chemistry · Chemistry key terms · required practicals · equations · Incandio Science · The 4CH1 specification